A new way of measuring the carbon in forests may help keep them from being cut down.

By Michael Lemonick

The news from the planet's forests has been surprisingly good lately, at least compared with the news of a decade or two ago. Globally, according to a United Nations report that came out last year, the rate at which forests are destroyed—logged or cleared to make way for farms or mines—was nearly 20 percent lower from 2000 to 2010 than it had been in the previous decade. Huge tree-planting programs, especially in China, reduced the net loss of forest even further. But vast areas are still being slashed, mostly in the tropics, including each year a Switzerland-size area of previously undisturbed, ecologically precious "primary" forest. Most of those trees are burned, and the carbon stored in their wood literally goes up in smoke. Rough estimates indicate deforestation still contributes around four billion tons of planet-warming CO2 to the atmosphere each year, an eighth of the human total.

If storing carbon in trees had an economic return, both climate and forests—and the tremendous diversity of life in forests—would be better off. At the UN-sponsored climate conference in Cancún, Mexico, last December, delegates reached agreement on a long-debated program known as REDD, short for Reducing Emissions from Deforestation and Forest Degradation. The basic idea is for rich countries to pay to preserve forests in poor ones, which can be cheaper than reducing their own carbon emissions.

One obstacle has been the lack of a reliable way of measuring how much carbon is stored in different forests without inspecting every acre from the ground. That's where the maps shown here fit in. They were made by a team led by Gregory Asner, a tropical ecologist with the Carnegie Institution. Satellites, Asner explains, can track forest loss, but not whether it's primary forest or secondary regrowth—whether it consists of huge, old-growth trees or of scrawny saplings that store less carbon.

Asner's team can now do that by surveying the forest from a plane with lidar, a laser-ranging instrument similar to radar. The device shoots an infrared laser pulse toward the ground 100,000 times a second and records how long it takes to reflect back. Some of the light bounces off the forest canopy, but some makes it all the way to the ground. "It's kind of like an MRI," says Asner. "We can figure out not just the height of the canopy, but also the 3-D structure of the forest," and thus how much carbon it's storing. You still need on-the-ground measurements to spot-check the lidar, Asner says, and satellites to get the big picture. But all three together just might offer a way of keeping carbon trading honest—and tropical forests intact.